Disclosed in Japanese Patent Laid-Open Publication No. 62-117539 is an example of a prior art ultrasonic diagnosing device in which to improve resolving power in a direction orthogonal to a scanning direction of sound rays. This type of ultrasound diagnosing device involves the use of an ultrasound probe having a configuration depicted in FIG. 8. Referring to FIG. 8, an ultrasound probe 1 generally designated at 1 is constructed of: an oscillator element group 2 arranged such that the elements are split at predetermined pitches in the sound ray scanning direction, i.e., a direction X, while in a direction Y orthogonal to the sound ray scanning direction the elements are three-split to thereby constitute three arrays, viz., element arrays 11, 12 and 13 in the direction X; an acoustic matching layer 3 attached to an ultrasound radiation face thereof; and an acoustic lens 4 assuming a semi-circular shape. Note that a direction Z in vertical to the ultrasound radiation face. The thus constructed ultrasound probe perfoms, when a Y-directional aperture reaches its maximum in the case of a target being positioned deep, an X-directional scan, i.e., a sector scan indicated by, e.g., a one-dotted line by employing all the element trains 11, 12, and 13. During this scan, a Y-directional width of the ultrasound beams is converged by means of the acoustic lens 4. Whereas if the target is positioned shallowly, a linear scan is, as indicated by a broken line, effected by using the central oscillator element 12 alone. At this time, the Y-directional aperture is smaller than in the deep position, and the Y-directional width of the ultrasound beams is narrowed corresponding to the small aperture. In the ultrasound diagnosing device adaptive to vary the Y-directional aperture, Y-directional resolving power is improved from the shallow level to the deep level. In the prior art ultrasound diagnosing device, however, a focal point of the acoustic lens is set typically in a deep position, which in turn leads to such a problem that the ultrasound beams can not sufficiently be converged in the direction Y in the vicinity of the acoutsic lens. Videlicet, the problem is that it is impossible to obtain the resolving power which is uniform in the direction Y over a wide range from the vicinity of the acoustic lens down to the deep position.